Superconducting materials are materials for which the electrical resistance becomes zero under some conditions. This is only possible if the superconducting material satisfies three conditions:                its temperature must be less than a critical temperature Tc;        it must be in a magnetic field weaker than a magnetic field Bc;        the current passing through the material must be less than a critical current Jc.        
Critical values are specific to each material. These three parameters, Tc, Bc and Jc, are dependent on each other, forming a critical surface. If the material is below the critical surface it is superconducting, otherwise it is resistive.
There are many superconducting materials, but only a small number of materials are suitable for manufacturing an electromagnet due to the small size of the critical surface. Niobium titanium (NbTi) is the most frequently used material at the present time due to its use in Magnetic Resonance Imagery (MRI) instruments and NMR (Nuclear Magnetic Resonance) spectrometers, which at the present time are the main industrial markets for superconductivity.
Niobium 3 tin (Nb3Sn) is another material used for very strong field NMR spectrometers, more than 10 Tesla. Magnesium diboride (MgB2), mixed bismuth strontium calcium copper oxide (BSCCO) and mixed yttrium barium copper oxide (YBaCuO) are other superconducting materials used for manufacturing electromagnets, but for the moment they are only used in research and development.
MgB2 has the advantage that it is inexpensive, it is very easy to use and its performance is better than NbTi at equivalent temperature for a magnetic field of less than 4 T, and the vast majority of MRI apparatuses operate with a magnetic field of between 1.5 T and 3 T. The increase in operating temperature means that “dry” cooling by conduction at between 10 and 20 K can be used instead of so-called wet cooling in a liquid helium bath at 4.2 K.
Two types of windings are widely used for the manufacture of electromagnets, and particularly superconducting magnets:                the solenoid winding that is a layer by layer winding;        a double pancake stack or double pancake winding, which is a turn by turn winding.        
Usually, a solenoid winding is used because it is easy to make, and is fast and inexpensive. However, a long individual conducting length is desirable for large systems, although this not always possible. In this case, i.e. a solenoid winding, junctions have to be made between the layers but this is not recommended for superconductivity.
Thus, a double pancake winding is often used to make superconducting coils at high and medium critical temperature because the industrially available individual conducting length (from 100 m to 4 km) is not sufficient to make a complete single piece coil generally requiring a few tens of km. Subsequently, a plurality of double pancakes is stacked to assemble the final electromagnet. A junction is made between each double pancake. This type of junction is easier to make than in the case of a solenoid winding because the junction is located on the outside radius of the electromagnet in a weak field zone.
In general, the double pancake winding method comprises the following steps:                the conductor length necessary for winding a double pancake is halved. Half of the conductor is transferred from a first reel to a second reel. Each reel thus contains the conducting length necessary for winding each pancake.        the reels are transferred onto a winding machine. One of the reels is installed on a tensioner, a system that imposes a tension in the conductor, for winding the first pancake. The second reel called the spare reel is installed above the winding table in order to kinetically tie the spare reel to rotation of the winding table to prevent the conductor on the spare reel from unwinding, while the first pancake is being produced.        set up the layer change and wind the first pancake;        once the first pancake has been wound, move the tensioner laterally and then transfer the spare reel onto the tensioner so as to wind the second pancake.        
This double pancake winding technique was initially developed for winding the NbTi conductor. However, it is difficult to apply for MgB2 conductors that, unlike NbTi conductors, are sensitive to deformations. MgB2 conductors have a maximum deformation threshold above which they loose their superconducting state. This limit is relatively low which imposes a high minimum radius of curvature of the conductor. Minimum radii of curvature for a standard conductor with a cross-section of 0.7*3.1 mm2 are 60 mm and 260 mm respectively.
It is also almost impossible to tell whether or not a conductor has been damaged during winding. Such a problem will only be observed when the magnet is finally put into operation, unless every double pancake is tested independently which is long and expensive. Since it is impossible for a superconducting magnet to be partially resistive, if there is a defect in a double pancake, then the magnet will have to be disassembled and the double pancake will have to be replaced. Thus, risks of conductor damage during winding must be limited.
The following steps are particularly critical for a fragile conductor such as MgB2:                operations to split the reels and transfer them onto the winding machine, the conductor between the two reels being free to move and therefore can be damaged;        shaping of the layer change in a special tool, the conductor is then turned back in the winding mandrel, the conductor is then free to move and can be damaged;        the operation to transfer the spare reel onto the tensioner for winding the second pancake because the conductor usually has to be unwound to be put on the tensioner. The conductor is also blocked at the exit from the layer change which creates a stress concentration point, therefore the conductor can easily be damaged.        
These problems are critical for superconductors, but they also exist for other conductor windings, regardless of whether or not the conductor is insulated and whether or not it is superconducting.